1. Field of the Invention
The present invention relates to a core barrel, particularly in the field of oil exploration, comprising, on the one hand, at least an outer tube and a coring bit which form an outer assembly and, on the other hand, at least one inner tube which, with a drilled core possibly present therein, and possibly with some of the formations under the base of the drilled core, forms an inner assembly, suspension and/or guidance means being provided to keep the inner assembly inside the outer assembly.
2. Background of the Prior Art
Numerous core barrels of this kind are known. Their suspension and/or guidance means form, between the outer assembly and the inner assembly, mechanical connections of the type involving a bearing, of the radial and step type, and of the double-acting thrust type. It is thus common practice to have at least one upper mechanical connection therein, often of the thrust-bearing type, at the rear end of the inner tube, when considering a direction of advance S of the core barrel into the ground, and to have a lower mechanical connection of the step bearing type (in the bit, for example) at the front end of the inner tube. There may also be intermediate mechanical connections, usually of the step bearing type, particularly when core drilling is to be performed in a position which is steeply inclined with respect to the vertical or even in a horizontal position.
In searching for a core barrel capable of supplying a drilled core sample which is as intact as possible and as representative as possible of the formations being probed, the person skilled in the art tends to make the mechanical connections between the abovedescribed outer and inner assemblies very severe, that is to say to make them have close tolerances, so as to obtain the most efficient possible guidance of the inner tube around the drilled core.
It has, however, been found that the drilled cores obtained could be greatly damaged as the result of the vibrations and jolts transmitted through the string of core-barrel control rods, by the phenomenon known as whirling of the coring bit in its hole, about its axis of rotation, and from the rotation of the latter about the axis of the hole, against the pseudocylindrical interior wall of the hole, which has a diameter greater than that of the bit, and by the catching of the latter in the formation that is to be sampled, etc., that is to say as the result of an unfavourable action of the outer assembly on the inner assembly.
The inventors of the present invention have resolutely chosen to go against the practice explained hereinabove and consider that the inner tube and the drilled core, which is received therein as core drilling progresses, need to be relieved of the stresses that the abovementioned outer assembly experiences during this same core-drilling operation and transmits to the inner assembly.
To this end, according to the invention, the suspension and/or guidance means are arranged in such a way that the inner assembly retains, at least at the location of these means, with respect to the outer assembly, a predetermined freedom of axial and/or radial and/or angular movement which is greater than usual.
As a result of this, it has thus been found, surprisingly, that harsh stresses on the outer assembly are no longer transmitted, or are transmitted to a very reduced extent, to the drilled core, whose integrity is only better preserved as a result.
It has also become apparent, as a result of this, that the material of the core barrel experiences markedly lower stresses and is better preserved than was the case in the past as regards destruction by impact, by the forcing of parts onto one another, and onto the drilled core.
According to one embodiment of the invention, the suspension and/or guidance means comprise at least one rotary thrust bearing for suspending and/or supporting the inner tube in the outer tube, this thrust bearing being mounted therein with a radial and/or axial and/or angular clearance that is greater than the usual clearance.
According to a particular embodiment of the invention, the suspension and/or guidance means comprise hydrostatic bearing means in which pressurized core-drilling fluid forms, during core drilling, one or more elastic cushions for the radial and/or axial and/or angular location.
According to an advantageous embodiment of the core barrel of the invention, means are arranged therein so as, during core drilling, and in collaboration with the effects of the pressure of the core-drilling fluid on the inner assembly, to keep the latter hydrostatically suspended within the outer assembly within the limits of the said freedom of movement and/or of the aforementioned clearance.
Other details and particular features of the invention will emerge from the secondary claims and from the description of the drawings which are appended to this text and which illustrate, by way of nonlimiting examples, some advantageous embodiments of the invention, which are depicted diagrammatically in axial section unless otherwise indicated, with cutaway, and possibly to different scales.